Quercus:
Hold in the CTRL key and right click on the Outlook icon in your system tray. This will show some advanced options in the context menu, one that will say "Test Email AutoConfiguration." Underneath the E-mail Address and Password text-boxes, there should be three checkboxes. Ensure that only the first one is checked, marked "Use AutoDiscover." Then hit the Test button.
This will verify that you have the IIS 7 autodiscover virtual directory configured properly, that your certificate contains a subject alternative name for the autodisover virtual directory if you require SSL, and that you have properly configured the Client Access URLs for internal/external access appropriately.
If you are able to connect to the autodiscover service, the test will load an XML file that contains the appropriate URLs and authentication requirements. If you can, send me a copy of the XML (dmccaffery@giodivine.com) for further analysis and we'll go from there!
Good Luck!
Quercus